As a waveguide for transmitting electromagnetic waves (high frequency signals) such as microwaves and millimeter waves, a waveguide disclosed in Japanese Patent Laid-Open No. Hei 11-284409 is known. The waveguide has: a pair of main conductor layers sandwiching a dielectric substrate; a group of through conductors for a pair of side walls formed so as to electrically connect the pair of main conductor layers; and a sub conductor layer electrically connected to the group of through conductors for side walls and formed between the pair of main conductor layers in parallel with the main conductor layers. It is considered that the waveguide is directed to electromagnetic waves in the TE mode on the basis of the description in the publication that “parts corresponding to an H plane and an E plane of a dielectric waveguide are formed by a main conductor layer and a group of through conductors for side walls” and “parts corresponding to an E plane and an H plane of a dielectric waveguide are formed by a main conductor layer and a group of through conductors for side walls”.
The inventors of the present invention are studying waveguides for electromagnetic waves in the TM mode. A waveguide for the TM mode can be formed by using the configuration disclosed in the publication. In the waveguide of the related art, however, a group of through conductors for side walls made of a number of through conductors for side walls have to be formed in a dielectric, so that the structure is complicated. There is consequently a problem that manufacturing cost is high.